Thermoplastic film, and in particular polyolefin materials, have been used for some time in connection with packaging of various articles including food products which require protection from the environment, an attractive appearance, and resistance to abuse during the storage and distribution cycle. Suitable optical properties are also desirable in order to provide for inspection of the packaged product after packaging, in the distribution chain, and ultimately at point of sale. Optical properties such as high gloss, high clarity, and low haze characteristics contribute to an aesthetically attractive packaging material and packaged product to enhance the consumer appeal of the product. Various polymeric materials have been used to provide lower gas permeability in order to reduce the transmission of oxygen through the packaging film and thereby retard the spoilage and extend the shelf life of products such as food items which are sensitive to oxygen.
Some cheese products are produced in such a way that the final cheese product emits a significant amount of carbon dioxide over time. In such cases, it is often desirable to provide a packaging material which is characterized by a relatively low oxygen transmission rate (i.e. good oxygen barrier), and a relatively high carbon dioxide transmission rate. A preferred O.sub.2 transmission rate is no more than about 500 cc/m.sup.2 -day-atmosphere (ASTM D 3985 at 73.degree. F.), more preferably no more than about 250 cc/m.sup.2 -day-atmosphere, and even more preferably no more than about 175 cc/m.sup.2 -day-atmosphere. A preferred CO.sub.2 transmission rate is at least about 750 cc/m.sup.2 -day-atmosphere at 730.degree. F., more preferably at least about 1000 cc/m.sup.2 -day-atmosphere, most preferably at least about 1200 cc/m.sup.2 -day-atmosphere. CO.sub.2 transmission is measured using an analytical technique analogous to ASTM D 3985. The ratio of CO.sub.2 transmission rate to O.sub.2 transmission rate is preferably greater than about 5:1, and most preferably at least about 7:1.
It is also often desirable to include in a packaging film a shrink feature, i.e., the propensity of the film upon exposure to heat to shrink or, if restrained, create shrink tension within the packaging film. This property is imparted to the film by orientation of the film during its manufacture. Typically, the manufactured film is heated to an orientation temperature and then stretched either in a longitudinal (machine) direction, a transverse direction, or both, in varying degrees to impart a certain degree of shrinkability in the film upon subsequent heating. After being so stretched, the film is rapidly cooled to provide this latent shrinkability to the resulting film. One advantage of shrinkable film is the tight, smooth appearance of the wrapped product that results, providing an aesthetic package as well as protecting the packaged product from environmental abuse. Various food and non-food items may be and have been packaged in shrinkable films.
For certain products the shrink force of the shrinkable film must be controlled in order to avoid deformation. Furthermore, for irregularly shaped products a heat shrinkable film must be able to draw into crevices without imploding. Thus, a sufficiently high tensile strength and percent elongation at or about the shrink temperature are required. Such properties are especially important for Swiss cheese packaging.